The majority of current 2G and 2.5G cellular networks operate across four frequency bands in the RF spectrum. These bands are globally standardized, which means that when a user travels with their mobile phone to different countries, the device’s RF section may need to support different frequency ranges. This necessitates a handset design that includes multiple configurations of parallel switches, power amplifiers (PAs), and surface acoustic wave (SAW) filters. However, thanks to mature technology, the established 2G market, and advanced integrated IC transceiver designs, these parallel architectures help reduce both cost and size.
Figure 1 illustrates the architecture of a typical quad-band GSM transceiver. Through integration, only three main components are required on the signal path: a multi-band PA and integrated switch module, a quad-band Rx SAW filter bank, and a silicon-based transceiver.
The situation for 3G is somewhat similar to earlier cellular systems. Initially, the UMTS 3G system was allocated one frequency band, making the RF front-end relatively simple and allowing the use of external filters without significant cost or PCB space. However, due to regional differences, regulatory restrictions, and new spectrum allocations, there are now 10 bands assigned to 3G networks worldwide, some of which overlap. As users expect their 2G phones to work internationally, 3G users also anticipate this capability, especially high-end users who rely on data and voice services. Thus, a more cost-effective multi-band transceiver design is essential. Figure 2 shows the current global band allocation. A modern multi-band handset should be able to cover at least three bands—low and two high bands.
One straightforward way to achieve multi-band functionality is by using a single-band transceiver design, where several front-end modules are implemented in parallel. The RF transceiver circuitry remains largely similar, but external matching and RF filters must be specifically tailored for each operating band. This approach works well with careful frequency planning, wideband PLL systems, and highly integrated sub-micron CMOS processes. However, considering chip cost, integration level, number of external components, and PCB area, this solution isn't ideal. An example of such a design is shown in Figure 3, though the GSM part has been omitted for simplicity.
Figure 1: An example of a quad-band GSM transceiver using just three devices.
Figure 2: Frequency allocation diagram for the 3GPP band.
Figure 3: Example of a 3G multi-band transceiver front end built from single-band designs.
The Othello-3 family of 3G transceivers uses an innovative processing architecture that overcomes the challenges of front-end integration. One major challenge is the need for external filters, as they define the input and output frequency ranges for a given band. In full-duplex systems like WCDMA, the transmitter and receiver operate simultaneously, requiring a duplex filter at the antenna to prevent high-power transmission signals from degrading the receiver's performance. Due to circuit limitations, additional separate filters are often needed on both transmit and receive channels, as shown in Figure 3.
Transmitter Architecture
A common superheterodyne transmitter architecture is illustrated in Figure 4. The filtered I/Q baseband input signal is mixed with an orthogonal local oscillator (LO) signal to produce a constant intermediate frequency (IF). After filtering to remove spurious and harmonic signals, the IF is then mixed with a variable frequency oscillator to generate the desired RF output. Gain control is distributed throughout the system. While an external SAW filter can simplify the design of the Tx channel, it also increases costs and PCB space.
The Othello-3 family employs a direct conversion or "zero heterodyne" transmitter architecture, directly upconverting baseband signals to the RF carrier. This eliminates the need for secondary mixing and associated RF parasitics, allowing the removal of external filters. However, without Tx filter attenuation, noise levels in the receiver band become critical.
To address this, the Othello-3 features a special modulator core that minimizes LO channel noise, traditionally far from the carrier. Its 86dB gain control is integrated into the modulator, reducing the need for additional circuitry and minimizing noise contribution. Low-noise design techniques are embedded throughout the transmitter, eliminating the need for extra filtering. Calibration is fully self-calibrating, ensuring optimal performance under all conditions. The modulator uses fully differential signal processing, and a built-in balun provides a 50-ohm single-ended output directly to the PA module.
The Othello-3 transmit channel also delivers excellent error vector magnitude (EVM) and adjacent channel leakage ratio (ACLR) performance. Removing the external SAW filter significantly reduces material costs and PCB space, enabling multi-band PA integration. As seen in the GSM market, all four bands can be included in a single PA package.
Direct Conversion Receiver
The direct conversion receiver architecture used in Othello transceivers is highly integrated and proven. In a GSM receiver, the entire receive chain can be integrated. For 3G systems, where transmitter signal leakage to the receiver is a concern, an interstage filter was previously required to reduce leakage during the critical mixing stage. While a SAW filter after the low noise amplifier (LNA) helps maintain sensitivity, it complicates integration.
Othello-3 includes three LNA modules—two for high bands and one for the low band. Each LNA has a single-ended input that easily matches its duplex filter. They also feature optimized bandpass responses to minimize out-of-band signals and transmitter leakage. The high linearity mixer design eliminates the need for external interstage filters, as shown in Figure 5. Receiver gain control is optimally distributed between the RF chain and baseband processing stages. Integrated gain distribution logic simplifies programming and calibration, allowing the baseband to set optimal gain with a single word.
Figure 5: AD6551 front-end architecture without an external filter.
This example highlights the integration of 3G RF devices, with vendors also exploring antenna switch modules that include all front-end modes and integrated band switches with GSM Rx SAW filter banks.
The ADI Othello-3 transceiver currently consists of the AD6551, ideal for WCDMA 3G handsets, and the AD6552, suitable for 3G TD-SCDMA mode. Both support 3GPP Release 5 and HSDPA operation.
Summary
Current single-mode 2G handsets can implement a quad-band RF solution using just three major chip packages, offering minimal size and necessary features. Early 3G handsets were limited to single-band designs, relying on external filters, but this wasn’t ideal for multi-band handsets in terms of size and cost. The Othello-3 eliminates the need for external filters, enabling further integration of front-end devices and PAs. With advancements in integration and switch design, more front-end components can be added gradually. With a transceiver like Othello-3, a fully optimized multi-mode architecture becomes possible.
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